1. Field of the Invention
This invention relates in general to tire pressure monitoring systems and more specifically to motion detection in tire pressure monitoring systems.
2. Description of the Related Art
Tire pressure monitoring systems (TPMSs) are utilized for monitoring the tire pressure of a motorized vehicle. Some TPMSs are located at the wheel and include circuitry for transmitting information to a central controller system of the motorized vehicle. Some TPMSs are battery powered, wherein the battery is required last up to 10 years.
In order to extend battery life, TPMSs may include two modes of operation, e.g. a motion mode and a park mode. In a motion mode, the TPMS may monitor tire pressure at a greater rate and/or transmit pressure readings to a central controller system at a greater rate than during the park mode. Typically, it is desired that the TPMS be in a motion mode when the vehicle is moving.
In the past, electrical mechanical motion switches and accelerometers have been utilized to provide motion detection in a TMPS. However, electrical mechanical switches may have mechanical reliability issues and may provide for an increased cost. Furthermore, such electrical mechanical switches may consume a relatively large amount of battery energy over the life of the TMPS due to pull up or pull down resistors implemented with the electrical mechanical switch.
Accelerometers have also been used for motion detection in a TMPS. Accelerometers detect motion by measuring centrifugal acceleration. However, some accelerometers consume a relatively large amount of power (e.g. as much as the pressure sensor) during operation and are relatively expensive.
Piezoelectric sensors have been used in motion detection for detecting wheel rotation, not vibration. One such device uses a physical member which makes contact with an under inflated tire, thus transferring a mechanical force to the piezoelectric sensor during each wheel rotation. A sensing system such as this has mechanical reliability issues, and can be easily damaged during tire mounting due to its proximity to the inner tire surface.
Piezoelectric sensors have also been used to detect wheel rotation by detecting gravity variations. However, these types of systems may have a poor low frequency response in the range of interest and may have a low output for ±1 G change in rotational force.
Piezoelectric sensors have also been used as accelerometers to detect the centrifugal acceleration in the rotating wheel, but there are disadvantages with these systems. Piezoelectric sensors do not effectively measure constant loads such as the centrifugal acceleration in a rotating wheel. A constant load may give a fixed charge output from the piezoelectric sensor which may dissipate quickly. To compensate for this issue, in some systems, the piezoelectric sensor is typically operated in a resonant mode which requires a circuit to drive part of the sensing structure into resonance for detection. However, this results in increased power consumption, thus negating the benefits of a motion detection system.
What is needed is improved motion detection system for a TPMS which consumes less energy.